1. Field of the Invention
This invention is directed to an apparatus and method for scanning an object for nitrogen to determine the presence of explosives. More particularly, the invention is directed to an apparatus utilizing neutron capture which causes gamma rays to be emitted from the object that are detected and analyzed to provide a representation of the distribution of the nitrogen contained within the object.
2. Description of the Prior Art
A significant threat to human life and property exists when an explosive device is concealed in luggage or parcels brought into buildings, aircraft, etc. As a result, there is a need by both the public and private sector of the country for a reliable technique for the detection of such explosive devices. As the threat of terrorist activities throughout the world, especially in airports, has increased, the demand for an efficient and practical device for scanning luggage to determine the presence of explosives has intensified.
It is well known that explosives may be detected by sensing the amount of nitrogen in the object being examined. One technique of detecting nitrogen is by netron capture. Neutron capture is premised on the concept that many elements emit gamma rays as a result of bombardment by neutrons and further that the gamma ray spectra resulting from such radiation exhibit properties and characteristics of the element.
U.S. Pat. No. 3,124,679 utilizes neutron capture to determine the nitrogen content in objects. The object is placed within a chamber where it is irradiated with neutrons and the resulting gamma rays are detected. Those gamma rays having energy characteristic of neutron interactions with atoms of the element under analysis are selected from all other radiations and their rate of occurrence is measured. This technique attempts to detect explosives solely on the basis of total nitrogen content and is therefore inadequate in view of the nitrogen content in everyday articles made of wool, silk, and leather. Since no discrimination can be made of elements having similar amounts of nitrogen this technique would not be useful in differentiating between explosives and other elements in luggage.
U.S. Pat. No. 3,832,545 to Bartko, is directed to a neutron capture system for detecting the presence of explosives incorporating an array of gamma detectors in order to provide a nitrogen profile of the object. The invention utilizes gamma detectors of approximately 8-12 inches in width which would provide a coarse profile thereby making the device impractical for detecting the presence and location of explosives in luggage. This is especially true in view of the plastic explosives which may be formed into an innocent shape. Moreover, Bartko recognizes that the ultimate sensitivity of the system depends on the level of thermal flux at the detection location. However, presently, there is no device for effectively maintaining an optimal thermal flux density within the radiation chamber.